1. Field of the Invention
The present invention relates to an objective lens actuator to be equipped in an optical pickup device which irradiates a light beam to an optical recording medium to enable recording and reading information, and, more particularly, to the configuration of an objective lens actuator having a plurality of objective lenses. The present invention also relates an optical pickup device having such an objective lens actuator.
2. Description of the Related Art
Optical recording mediums, such as a compact disc (hereinafter called “CD”) and digital versatile disc (hereinafter called “DVD”), are popular. Further, studies have recently been made on enhancement of the density of optical recording mediums to increase the amount of information thereof, thereby putting optical recording mediums, such as HD-DVD and Blu-ray Disc (hereinafter called “BD”), which can record a huge amount of information to practical use.
An optical pickup device is used in reading information from such an optical recording medium or writing information thereon. Depending on the type of the optical recording medium, it is necessary to change the numerical aperture (NA) of an objective lens used in an optical pickup device and the wavelength of a light source used therein. For example, an objective lens whose NA is 0.45 and a light source whose wavelength is 780 nm are used for a CD, an objective lens whose NA is 0.60 and a light source whose wavelength is 650 nm are used for a DVD, and an objective lens whose NA is 0.85 and a light source whose wavelength is 405 nm are used for a BD.
Because the NA of an objective lens in use and the wavelength of a light source in use vary depending on the type of an optical recording medium, different optical pickup devices may be used for different optical recording mediums. It is however preferable that a single optical pickup device can compatibly use plural types of optical recording mediums, and multiple optical pickup devices of such a type have been developed. Some of such optical pickup devices have a plurality of objective lenses which can be switched from one to another depending on the type of an optical recording medium in use.
An objective lens included in an optical pickup device is generally mounted on a lens holder provided in an objective lens actuator, so that the focus direction and tracking direction can be adjusted. The same is true of the optical pickup device that has a plurality of objective lenses, which however should be mounted on the lens holder in such a way that the objective lenses do not tilt with respect to one another (i.e., the center axes of the objective lenses become in parallel).
The tilt angle of an objective lens actuator having a plurality of objective lenses is normally adjusted using a dominant one of the objective lenses for the optical pickup device so that the influence of comatic aberration or the like is reduced. At the time of, for example, adhering the objective lenses to the lens holder, however, the adhesion angle may be deviated, causing the objective lenses to tilt in relative to one another (relative tilt). In this case, when an objective lens which has not been used in adjusting the tilt angle of the objective lens actuator is used, comatic aberration occurs, bringing about a problem of degrading the quality of information read by the optical pickup device.
In an objective lens actuator having a plurality of objective lenses, therefore, a relative tilt caused between the objective lenses needs to be suppressed. There is a conventional proposal on such a technique. For example, JP-A-2005-174485 discloses a technique of designing objective lenses of an objective lens actuator in such a way as to be able to adjust the swing of the objective lenses about the principal point of the objective lenses and configuring an objective lens holding cylinder which can secure an objective lens by an adhesive or the like in such a way as to be able to adjust the swing of the objective lens holding cylinder about the principal point of the objective lenses. This configuration can reduce a relative tilt between the objective lenses.
However, the configuration disclosed in JP-A-2005-174485 requires special processing on the objective lenses to reduce a relative tilt, thereby undesirably increasing the cost of preparing the objective lenses. Because the objective lenses are configured to move along a spherical surface or the like in case of adjusting the tilt angle of each objective lens to reduce the relative tilt between the objective lenses, it is not quite easy to smoothly move the objective lenses, which makes the adjustment work harder.
As a solution to this problem, as shown in FIG. 7A and FIG. 7B, an objective lens 100 to be mounted on a lens holder 101 is configured to be mounted on a tilt adjusting holder 102 which has a structure to facilitate tilt adjustment, thereby ensuring low-cost adjustment of a relative tilt with better workability. This configuration however has the following problem. FIGS. 7A and 7B are diagrams for explaining the configuration of a conventional objective lens actuator; FIG. 7A is a diagram showing the cross sections of a part of the lens holder 101 of the objective lens actuator, and the tilt adjusting holder 102 on which the objective lens 100 is mounted, and FIG. 7B is an enlarged view of a portion encircled by a dotted line in FIG. 7A.
When adjustment of a relative tilt which is executed by moving the tilt adjusting holder 102 is completed, the tilt adjusting holder 102 is securely adhered to the lens holder 101. In consideration of the workability or the like, normally, the tilt adjusting holder 102 is moved after a UV adhesive which is cured by UV irradiation is applied to a corresponding portion of the tilt adjusting holder 102 or the lens holder 101 (for example, a filled-in-black portion 103 in FIG. 7B), not that an adhesive is supplied after the state of securing the tilt adjusting holder 102 is determined. Then, when adjustment of the relative tilt is completed, UV irradiation is performed to cure the adhesive, thereby securing the tilt adjusting holder 102.
In this case, the adhesive is likely to run around to other locations than an adhesion portion 103 at the time of adjusting the relative tilt. An adhesive 104 running around from the adhesion portion 103 has a certain thickness. When the temperature around the objective lens actuator changes, for example, the tilt adjusting holder 102 or the like is shifted from the set position due to the influence of thermal expansion or the like. In this case, the relative tilt between the objective lenses increases, thus reducing the reliability of the objective lens actuator.